A vacuum pump disclosed in, for example, Japanese Patent No. 3974772 has been known as a vacuum pump for exhausting gas from a chamber by means of the rotation of a rotor. In this vacuum pump, the entire rotating body that has the rotor (8) and rotor blades (10) provided integrally with an outer circumferential surface of the rotor (8) needs to be balanced during the assembly stage of the vacuum pump.
Especially in this vacuum pump of Japanese Patent No. 3974772 that exhausts a corrosive gas, since a corrosion protection membrane such as a nickel-phosphorus plated coating is formed on the surface of the rotor (8), the entire rotating body is balanced while preventing corrosion of the rotor (8) by the application of a synthetic resin adhesive as the mass adding means to the region of the surface where the corrosion protection membrane is formed (see paragraph 0008 and FIGS. 1 to 3 in Japanese Patent No. 3974772).
In regard to this type of vacuum pump, the following configurations have been known to further improve the evacuation performance: a configuration in which a part of a rotor made of a metallic material such as an aluminum alloy is made of a material such as a fiber-reinforced resin that is lighter and stronger than the metallic material (see Japanese Patent Application Publication No. 2004-278512, for example), and a configuration, such as that of the conventional vacuum pump (the threaded groove pump parallel flow type) shown in FIG. 9 of the present application, in which threaded groove exhaust flow passages R1, R2 are arranged in parallel in order to exhaust gas by means of the rotation of a rotor 6 (see Japanese Patent No. 3971821, for example).
However, according to the conventional vacuum pump (the parallel flow type) shown in FIG. 9 of the present application, the downstream region from substantially the middle of the rotor 6 (a connecting portion 60, to be precise) (between substantially the middle of the rotor 6 and an end portion of the rotor 6 at a gas outlet port 3 side) functions as a threaded groove exhaust portion Ps. This region of the threaded groove exhaust portion Ps is provided with the threaded groove exhaust flow passages R1, R2 on the inner and outer circumferences of the rotor 6 to make the threaded groove exhaust flow passages parallel and achieve further improvement of the evacuation performance. Therefore, applying the conventional balancing technique of Japanese Patent No. 3974772 to the conventional vacuum pump (parallel flow type) shown in FIG. 9 of the present application creates the following problems.
As shown in FIG. 9 of the present application, a synthetic resin adhesive M1 is applied to the inner circumferential surface of the rotor 6 opposing an inner threaded groove 19A, to obtain a balancing portion BC, which makes the effective thread length of the entire threaded groove exhaust portion Ps short, deteriorating the evacuation performance of the vacuum pump P6.
As shown in FIG. 9 of the present application, the balancing portion BC formed by the application of the synthetic resin adhesive M1 is exposed to the threaded groove exhaust flow passage R1 on the inner circumference side of the rotor 6, and the exposed synthetic resin adhesive M1 is exposed to the corrosive gas contained in the threaded groove exhaust flow passage R1. Consequently, the synthetic resin adhesive M1 for achieving the balance breaks into fragments due to corrosion thereof, which possibly end up flowing out to the process chamber or other closed chambers of the manufacturing apparatuses described above. The reason that the synthetic resin adhesive M1 flows out can be because, for example, kinetic energy of the rotary motion of the rotor acts on the fragments or because the exhaust gas flows back from the vacuum pump to the chamber. This flow of the fragments similarly occurs when mass adding means other than the synthetic resin adhesive M1 is employed as a weight to achieve the balance.
Especially when a balancing groove D is formed on the inner circumferential surface of the rotor 6 and the synthetic resin adhesive M1 is applied to the groove D in the specific configuration of the balancing portion BC of the rotor 6 as shown in FIG. 9 of the present application, the fragments of the synthetic resin adhesive M1 that are caused by corrosion might accumulate in the groove D instead of immediately falling off the balancing groove D. Therefore, when the fragments of the synthetic resin adhesive M1 that are created by experimental corrosion accumulate in the groove D in an anti-corrosion test of the vacuum pump, such fragments cannot be observed during the anti-corrosion test, and as a result the fragments flow out from the delivered vacuum pump to the upstream apparatus.
In addition, when applying the synthetic resin adhesive M1 to the balancing groove D described above, first, the synthetic resin adhesive M1 is applied first to a tip end of a rod-like tool T, and then the tip end of this tool T is inserted into a gap L between a rotor shaft 5 and the rotor 6, as shown in FIG. 10 of the present application (see the tool T indicated by the double broken line in FIG. 10). In so doing, due to the predetermined depth of the balancing groove D from the inner circumferential surface of the rotor 6, the synthetic resin adhesive M1 cannot be applied to the groove D unless the tool T inserted as described above is inclined at a predetermined angle with respect to the inner circumferential surface of the rotor 6 (see the tool T indicated by the solid line in FIG. 10), resulting in a contact/interference of the tilted tool T with the rotor shaft 5, hence poor balancing workability. Especially when the vacuum pump is small, the tilted tool T easily comes into contact with or interferes with the rotor shaft 5 due to the narrow space between the rotor shaft 5 and the rotor 6, resulting in poorer balancing workability.